1. Field of the Invention
This invention pertains to the measurement of cable characteristics and more particularly relates to an apparatus for determining the relative phase shift between transmission lines and for achieving a phase match therebetween.
2. Description of the Prior Art
In the prior art, phase measuring methods were essentially laboratory techniques which yielded good results for transmission lines that were physically or electrically short. In recent years, however, the need to assure the phase match of long cables installed in aircraft has arisen, creating new problems. The great length of such cables has required that the physical separation of the cables' ends be large, often necessitating either remote responding instrumentation, or technicians located at each end of the transmission lines in communication with each other. Often one end of a cable is inaccessible, rendering location of a technician at that end inconvenient. Additionally, the cramped quarters of an aircraft's interior requires that, regardless of the solution adopted, the physical size and number of the instruments used should be minimized. One method used in the prior art, is an adaptation of a laboratory phase bridge. In this method, power from a signal generator is divided in a power divider and fed to each transmission line under test, the free end of each transmission line being connected to a conventional phase bridge. The operator can then adjust phase shifters and attenuators within the phase bridge to achieve a null condition. Phase match is indicated by noting the adjustments needed to achieve this condition.
In addition to the previously mentioned problems, the phase bridge technique suffers from several other drawbacks. Automatic operation is difficult to provide over reasonably broad frequency ranges and the process is difficult and tedious if the nulling operation is performed manually. Furthermore, the electrical instability of long cables and connectors in the system can be a major problem because it is difficult to insure that all connections are tight and stable in a method as slow as the phase bridge technique.
A better solution previously in use utilizes a superheterodyne receiver technique. In this method, a signal is applied to one end of the cables under test while the other ends are connected to the input terminal of a dual receiver, wherein, the signals are mixed with a local oscillator thereby producing IF signals for each input. Since a common local oscillator is employed for producing the IF signals, the relative phase between the cables is maintained at the IF frequency and can be readily detected. This technique, while adaptable to automation and remote operation, retains the problems of physical bulk resulting from the method's complexity, and cable instability. Additionally, if operation over a broad range of frequencies is required, the necessity for good phase symmetry in the power divider and receiver can be cost prohibitive.
The present invention provides a simple method for obtaining the desired phase match information without the requirement for instrumentation at both ends of the cable.